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Transformer proximity effect: Pri and Sec interaction? (aka Interleaving?)

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cur8xgo:
I am trying to grasp whats going on with proximity effect in the case of multiple windings interacting.

For instance as described here: http://ridleyengineering.com/images/phocadownload/13%20proximity%20loss.pdf

I think I get it when we are talking about one winding by itself. Conductor thickness / skin effect ratio, use Dowell equation for total layers, voila Rac/Rdc ratio.

However, I don't really have a mental model for how multiple windings are interacting, for instance primary and secondary in a two winding transformer.

I see all the ingredients are there for interaction and optimization..currents going in opposite directions, gains from reducing layers, interleaving windings, etc...

And that document describes interleaving.

However its not sinking in. I am missing something.

In my dream world they do not interact at all and I can just calculate the Rac/Rdc ratio of the secondary as if the primary isn't there.

And in this way optimize the design for proximity effect for both primary and secondary.

Or I suppose, my dream world could exist if the windings are just not overlapped and are on different parts of the core. (true I think?)

T3sl4co1l: I know you recommended an interleaved design and I wish I understood how it works but I don't yet.

MagicSmoker:
Oversimplifying somewhat, proximity loss is the same as skin effect (which itself is the product of eddy currents), except the source of the alternating magnetic fields is adjacent winding layers in which current direction/polarity is the same. As each new layer is added to a winding, the magnetic fields from all of the layers in it sum together in stepwise fashion. Hence you want to minimize the number of layers.

If a winding layer has current flowing in the opposing direction, as happens when you go from primary to secondary in a forward converter, then the magnetic fields cancel each other out, also in stepwise fashion from the point of meeting. Thus, you can also minimize proximity loss by alternating between primary and secondary each layer if you must have more than one layer for each winding. Alternating layers, or interleaving, also reduces leakage inductance, so would seem to be a win-win all around, except that each time you change from primary to secondary (or vice versa) you might need to wind 1-3 layers of insulating tape to meet safety agency requirements, and you will also need to bring these layers out to a pair of pins on the bobbin, so there are practical limits to the number of alternating layers which can be accommodated. Also, interleaving increases the stray capacitance between the primary and secondary which makes common mode noise worse. Lots of nuances involved here...

...quite a few of which I am leaving out, but if all you remember is that you can minimize proximity effect first by minimizing the layer count (even if that means reducing the wire diameter, which might seem counterintuitive at first blush) and second by interleaving the primary and secondary, where space, bobbin pin count, stray capacitance and labor cost to wind the transformer permit (them being some of the nuances involved here).

cur8xgo:

--- Quote from: MagicSmoker on June 11, 2019, 08:01:34 pm ---Oversimplifying somewhat, proximity loss is the same as skin effect (which itself is the product of eddy currents), except the source of the alternating magnetic fields is adjacent winding layers in which current direction/polarity is the same. As each new layer is added to a winding, the magnetic fields from all of the layers in it sum together in stepwise fashion. Hence you want to minimize the number of layers.
--- End quote ---

Okay to try and make sure my thinking is consistent: it should not surprise me that the field from each turn add like this. After all, thats exactly what one designed the winding to do in the first place, although the goal was to have that field confined to the core. The field outside the core is the leakage field of the transformer, correct?

To validate that idea, if you designed a transformer with the same turns for pri and sec, and wound both windings simultaneously in a single plane (multiple layers though), with, lets say foil so its easier to imagine, would you end up with a zero leakage inductance transformer? And, simultaneously, zero proximity effect transformer? I would think skin effect would still be present though.


--- Quote ---If a winding layer has current flowing in the opposing direction, as happens when you go from primary to secondary in a forward converter, then the magnetic fields cancel each other out, also in stepwise fashion from the point of meeting. Thus, you can also minimize proximity loss by alternating between primary and secondary each layer if you must have more than one layer for each winding. Alternating layers, or interleaving, also reduces leakage inductance, so would seem to be a win-win all around, except that each time you change from primary to secondary (or vice versa) you might need to wind 1-3 layers of insulating tape to meet safety agency requirements, and you will also need to bring these layers out to a pair of pins on the bobbin, so there are practical limits to the number of alternating layers which can be accommodated. Also, interleaving increases the stray capacitance between the primary and secondary which makes common mode noise worse. Lots of nuances involved here...

...quite a few of which I am leaving out, but if all you remember is that you can minimize proximity effect first by minimizing the layer count (even if that means reducing the wire diameter, which might seem counterintuitive at first blush) and second by interleaving the primary and secondary, where space, bobbin pin count, stray capacitance and labor cost to wind the transformer permit (them being some of the nuances involved here).

--- End quote ---

The thing I don't get here, is why do you have to alternate? Why can't you just wind them simultaneously the whole way (or until one winding has all its turns)?

Also my model is breaking down when I try to figure out why the current density (proximity effect) increases step wise for each layer.

I see the winding as creating one big summed field. The only reason I can see current decreasing in each layer is that somehow the field from each layer can only affect the conductor immediately next to it. Why would that be true though?  Cant the field from layer 1 reach out and influence field 15?

So trying to imagine this..I realize the system has to be analyzed at an "operating point" and can't be looked at as cause and effect for each layer.

So if we say the winding has a certain current through it, obviously, the same for all layers, the outer layer is going to have the worst current density because.....why?

BTW Colonel McLymans Transformer and Inductor Design Handbook has a section on proximity effect and my instinct tells me the answers are there..I just can't seem to make it all fit in my head.



cur8xgo:
This picture is probably explaining everything I just dont get it though

"Flux does not penetrate conductors"?? Aren't the currents going in the same direction?


cur8xgo:
Here too..I have been just reading and re-reading this. I have to figure out why this isn't sinking in.

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